A frequent late effect of radiotherapy of cancer is radiation induced fibrosis in normal tissue which must be exposed during treatment. The induction of fibrosis often is dose-limiting and thus reduces probability of tumor control. This study will define fibrosis in selected normal tissue of the mouse (liver, kidney, lung, central nervous system) as related to radiation dose, quality, fractionation, and time of development. The relation between fibrous tissue formation and radiation damage to the vascular system will also be studied.